In the drywall construction field two general types of corner trim strips have been commonly used, the "nail-on" type and the "tape-on" type. Nail-on beads commonly take the form of an angle strip of metal with side flanges meeting at a center corner rib providing shoulders against which spackle or joint cement can be dressed when feathered from the adjoining wall surfaces to cover the edges and outer faces of the side flanges and the heads of the nails or other fasteners securing these flanges to the wall structure. Tape-on trim strips typically utilize paper wings to secure a metal corner angle in position rather than using fasteners. These wings are lateral extensions of a paper cover strip which is bonded to the metal angle, usually on the outer faces of the side flanges. Spackle or joint cement for dressing the corner normally adhere significantly better to the paper cover strip of tape-on beads than to the exposed metal of nail-on beads.
U.S. Pat. No. 2,649,890 (Dunlap et al.) discloses a machine and method for making tape-on trim strips for drywall corners of the type comprising a vee-shaped metal core strip covered by a wider flexible tape extending as flexible wings beyond the longitudinal edges of the core strip. The core strip is described as being preferably between 0.015 and 0.020 gauge and the tape is indicated as preferably being a fairly heavy weight of kraft paper. Such a trim strip is shown and described in Australian Patent No. 153,625 (Dunlap), published May 22, 1952. As previously indicated, this is the type of reinforcing strip which has been widely commercially used as a tape-on corner reinforcing strip, except that for several years, the elbow portion of the metal core strip has been formed with a longitudinal rib covered with paper as shown, for example, in U.S. Pat. No. 5,131,198. As in the case of typical nail-on corner trims, this paper over metal rib provides shoulders against which spackle or joint cement can be dressed when feathered from the adjacent wall surfaces over the paper covering the core strip. From time to time alternative tape-on corner reinforcing strips have been developed to be adjustable so as to also fit other than right-angle corners or to be lighter in weight or supposedly cheaper to produce.
For example, U.S. Pat. No. 2,862,264 (Perna) discloses an adjustable corner reinforcing unit for drywall corners in which a relatively wide carrier strip of kraft paper or a textile fabric is secured by adhesive to a pair of relatively narrow reinforcing strips of sheet metal which are preferably about 0.015 inch thick and spaced apart about a sixteenth of an inch. The Perna product may be folded to a right angle shape along a longitudinal folding line extending through the space between the reinforcing strips. When so folded the carrier strip bends over the void between the reinforcing strips and so the carrier strip is the only support at the corner.
U.S. Pat. No. 3,109,207 (Cooper) aims to provide a lighter and cheaper corner reinforcing strip for drywall corners. This reinforcing strip has a fabric strip fixed by adhesive between two paper tapes of different widths. A central length of relatively rigid wire is held by adhesive between the fabric strip and the wider of the two tapes. When the Cooper reinforcing strip is bent to a right angel configuration with the wire at the corner and the wider paper tape at the outside of the corner, the outer half of the wire and the adjoining portion of the wider tape form a corner rib. The Cooper product is preferably bent to the angle shape during the manufacturing of the product. However, the patent indicates that the product may be merchandised in a flat condition and bent later.
Although the above-described corner reinforcing strips have been known for over 35 years they and the remaining prior art have not sparked the development of an adjustable tape-on corner trim strip having a well-reinforced corner rib and which is economical to produce.